Among solid-state imaging devices, there is one capable of using a so-called electronic shutter function (an electric charge sweep operation in a substrate direction) to sweep a generated electric charge in the substrate direction by superimposing an electronic shutter pulse on a substrate voltage. For example, the substrate voltage can adjust a saturation signal amount of a light-receiving part. By changing the substrate voltage in a drive mode, the saturation signal amount necessary in each drive mode can be adjusted. A signal can be reset by application of a shutter pulse to the substrate voltage. This enables adjustment of an exposure time. Here, to drive a solid-state imaging device, a driver (pulse drive unit) is used. Both of the solid-state imaging device and the driver (especially, a vertical driver) require a voltage value away from an operation voltage value (for example, +5V system, +3V system, +1.8V system, and the like) required in other various circuits, and usually, two voltages having different polarities are necessary (because a reference potential is 0 V). Therefore, these two types of voltage are increased/decreased and supplied by a voltage conversion circuit (DC/DC converter, or the like), for example.
Here, in a driver capable of performing an electric charge sweep operation in the substrate direction, the electric charge sweep control is performed by an electronic shutter pulse driver circuit in the driver in order to control a timing of the start of storing an electric charge of a photoelectric conversion part such as a photodiode, and the electronic shutter pulse is applied to a first polarity substrate (for example, an n-type substrate) through a separate wiring from a transfer clock. A substrate voltage control circuit is connected to a substrate voltage terminal of the first polarity substrate, and a predetermined voltage (reverse bias voltage) is always applied to the first polarity substrate during the operation. At the sweep of an electric charge, the electronic shutter pulse driver circuit outputs a pulse signal, and a voltage of the pulse signal is superimposed on an applied voltage of the substrate voltage control circuit, a reverse bias voltage stronger than usual is applied to the first polarity substrate, and the stored electric charge is swept.
Meanwhile, when such a solid-state imaging device is driven, a voltage range applicable in each terminal or between terminals is defined as an absolute maximum rating. Such a rating is observed in a steady state. However, for example, the rating may deviate from the rating when a power supply switch of the imaging device is turned ON/OFF, or the like. Therefore, depending on a configuration of the electronic shutter pulse circuit between the driver and the solid-state imaging device, an abnormal voltage exceeding the absolute maximum rating may be applied to the substrate voltage terminal at the start or the interruption of a power supply, and there is a concern that the solid-state imaging device may be deteriorated or broken.
As a measure, for example, Japanese Patent Application Laid-Open No. 10-327360 discloses a protection circuit in which a clamp circuit including a DC cut capacitor, a clamp diode, and a discharge resistor is formed into a two-stage configuration. The diode is connected to the resistor in parallel, and an anode end is connected to a reference potential (usually, 0 V). That is, the diode is connected to be conducted by a negative voltage of a substrate voltage terminal. A connection point of the capacitor, a cathode end of the diode, and the resistor of a clamp circuit in the second stage is connected to the substrate voltage terminal. Accordingly, when a potential of the substrate voltage terminal exceeds a forward drop voltage of the diode at the start or the interruption of a power supply (in reality, becomes larger toward the negative potential side), the diode is turned ON. Therefore, the potential of the substrate voltage terminal is clamped to nearly the forward drop voltage of the diode. This can prevent a negative voltage that falls below the rating from being provided to the substrate voltage terminal.